loose cannon

loose cannon

noun
a person whose reckless behavior endangers the efforts or welfare of others.

Origin:
1975–80

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Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
loose cannon
 
n
a person or thing that appears to be beyond control and is potentially a source of unintentional damage

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009
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American Heritage
Idioms & Phrases

loose cannon

One who is uncontrolled and therefore a serious and unpredictable danger. For example, We can't trust her to talk to the pressshe's a loose cannon. This metaphoric expression alludes to cannon mounted on the deck of a sailing ship, which if dislodged during combat or a storm could cause serious damage to both vessel and crew by sliding about. Its figurative use dates from the first half of the 1900s.

The American Heritage® Dictionary of Idioms by Christine Ammer.
Copyright © 1997. Published by Houghton Mifflin.
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